The present invention relates generally to methods and apparatus for testing HDTV systems, and more particularly to a method and apparatus for testing an HDTV signal condition, HDTV terrestrial reception installation.
HDTV broadcasting standards for the United States were determined by the Advisory Committee on Advanced Television Service (ACATS). The transmission standard that was selected uses a vestigial sideband (VSB) with a symbol level of eight. Video and audio signals compressed into MPEG-2 bitstreams are multiplexed for broadcast. FIG. 1 shows a single segment of the 8VSB format. Each segment contains 832 symbols that include an MPEG-2 data packet having 187 bytes of data as well as 21 bytes of overhead.
In broadcasting, the signal is transmitted through free space and is thus exposed to various sources of noise. Multipath propagation generates a signal distortion which contributes to the noise level. To increase the signal's immunity to noise, Trellis coded modulation is used and Reed-Solomon parity coding is added for error-correction decoding. In addition, during the final stage of encoding, the data is randomized to avoid broadcasting long strings of 1s or 0s, which would result in a DC bias and would result in the loss of synchronization at the receiver.
All-digital HDTV transmission systems deliver about 20 Mb/s and by their nature have a very sharp threshold below which no reception at all is obtained. This sharp threshold is due to the rapid increase in the Bit Error Rate (BER) with a decreasing signal-to-noise ratio. Experimental testing in the field and laboratory has demonstrated that this threshold is about 15.16 db for a BER of 3.times.10.sup.-6.
A useful measurement in determining the quality of a received signal is the noise margin, i.e., how much of a signal-to-noise degradation can be tolerated without exceeding the specified bit-error rate below which there is no reception. That is, the greater the noise margin, the better the signal quality. The noise margin can be used, for example, when evaluating antenna patterns for HDTV reception and when the antennas are installed and oriented in the field. Field operators can ensure that installation and setup is complete when the noise margin is maximized.
Known techniques for measuring the noise margin of a signal involve the use of a random or "white" noise source, which is a complex and expensive device, thus making it impractical for use in a portable device intended for use in the field during installation of antennas and receivers.
Accordingly, the present invention is directed to the problem of providing a simple and inexpensive method and apparatus for determining the noise margin of a digitally broadcast signal such as an HDTV signal.